1. Field
The present invention relates to the field of determining the parameters of particles, droplets, bubbles or the like, and more particularly, to the determination of particle size and velocity using laser light scattering.
2. Art Background
There has long been a need to measure particle, droplet, and bubble size in sprays, planetary atmospheres, combustion processes and the like. Such measurements are useful in aircraft icing studies, planetary studies, fuel analysis, and numerous combustion and nozzle applications. A number of techniques employing laser light scattering have been developed to determine the size and/or velocity of particles, droplets, bubbles or the like.
In one system, commonly referred to as a particle sizing interferometer, a pair of laser beams of equal size and intensity are caused to cross and form a sample volume. An interference pattern between the beams is established by this crossing, and particles passing through this volume scatter light in proportion to the spatially varying light intensity within the interference pattern. From the scattered light, information concerning both the particle's size and velocity can be computed. The particle size is determined from a visibility factor (V) which is a function of the relative maximum and minimum intensity of the received scattered signal. A more detailed explanation of this interferometric technique is given in by the inventor W. D. Bachalo in an article entitled "A Method For Measuring The Size and Velocity of Spheres By Dual Beam Scatter Interferometry", Applied Optics, Vol. 19, Feb. 1, 1980; and U.S. Pat. No. 4,329,054 which issued on May 11, 1982. However, the determination of particle size based on a visibility function may give rise to significant errors. Signal visibility used for determining a particle's size requires that complete interference of the laser light occur at the measurement volume. In order to accomplish this, the laser light source must have a high degree of coherence, the beams must be of equal intensity and polarization, and they must be exactly parallel and overlap identically at the beam cross-over defining the interference pattern. In addition, other droplets passing through the pattern may cause beam extinction during measurements of a particular particle's size and/or velocity. Moreover, prior art visibility based size measurements are generally limited to a size range of one decade or less.
An alternate approach to the visibility method was described by F. Durst and M. Zare, in a paper entitled "Laser Doppler Measurements in Two Phase Flows", Proceedings of the LDA Symposium, Copenhagen, 1975. In that work, it was recognized that the interference patterns reflected and refracted by particles, droplets, bubbles or the like could be related to the size of the particles. The Zurst model assumes that there are no fringes generated where the two laser beams cross, but rather, that the interference fringes are formed by the scattered radiation from a particle, droplet or the like. The basic analysis provided by the authors illustrated that the shape and spacing of the fringes produced by reflection and refraction are functions of the angle between the incident laser beams, their wavelengths, as well as the direction of light collection and particle size. The authors derived formulas using a simple approach which are valid for small beam intersection angles, and large distance to observation and sphere size ratios. Although the authors claim that spherical particles could be measured using a double photo-detector apparatus, they later recognized that the size measurements required that the distance between the photo-detectors be matched to the fringe distance, and thus, to the particle size to be measured. They concluded that this requirement was a disadvantage for practical measurements of size distribution and rendered the method useful only for the measurement of size variations (e.g., the growth of a bubble). Moreover, the authors concluded that optical size measurements should be based on a direct recording of the fringe size using, for example, a vidicon tube, a multi-channel analyzer and a oscilloscope.
The proposed approach of Durst and Zare using a direct imaging means, such as a vidicon tube, encumbers the method with unnecessary information, slows the ultimate data rate of the system, and requires more extension software to extract the Doppler period and fringe spacing information. In addition, a further problem not acknowledged by Durst and Zare was the fact that their optical arrangement could not be used where the particle's trajectory is random as is the case of actual sprays. When the proposed photo-detectors are used without receiver lenses and apertures, scattered light from any point along the laser beam will reach the detector. However, in order to be functional, the two detectors must receive light simultaneously from only one spherical particle at a time.
As will be disclosed, the present invention provides apparatus and methods which permit particle sizing based on the phase variations of the scattered light caused by a particle, droplet, bubble or the like passing through an interference pattern. In addition, the present invention operates independently of the optical assumptions regarding the formation of interference fringes, and measures particles unambiguously over the wide range of sizes and number densities typical in spray and bubble environments.